Physics 2b- Electricity and the Atom

Current and Potential Difference

Current: is the flow of electric charge round the circuit. Current will only flow through a component if there is a potential difference across that component. Unit: ampre, A

Potential Difference: is the driving that pushes the current around. Unit: volt, V

Resistance: is anything in the circuit which slows the flow down. Unit: ohm, Ω

The greater the resistance across a component, the smaller the current that flows ( for a given potential difference across the component)

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Total Charge Through a Circuit Depends on Current

Current is the rate of flow of charge. When current (I) flows past a point in a circuit for a length of time (t) then the charge (Q) that has passed is given by this formula:

Current is measured in amperes (A)

Charge is measured in coulombs (C)

Time is measured in seconds (s)

More charge passes around the circuit when a bigger current flows.

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Example of Current= Charge/Time

A battery charger passes a current of 2.5 A through a cell over a period of 4 hours. How much charge does the charger transfer to the cell altogether?

Q= I x t = 2.5 x (4 x 60 x 60) = 36 000 C (36 kC)

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Potential Difference is the Work Done

--> The potential difference (or voltage) is the work done ( the energy transferred, measured in joules, J) per coulomb of charge that passes between two points in an electrical circuit. It's given by this formula:

P.D= Work Done Charge

--> So, the potential difference across an electrical component is the amount of energy that is transferred by that electrical component (e.g. to light and heat energy by a bulb) per unit of charge.

--> Voltage and potential difference mean the same thing.

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Circuit Symbols

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The Standard Test Circuit

This is the circuit you use if you want to know the resistance of a component. You find the resistance by measuring the current through and the potential difference across the component.

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The Ammeter

Measures the current (in amps) flowing through the component.

Must be placed in series.

Can be put anywhere in series in the main circuit, but never in parallel.

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The Voltmeter

Measures the potential difference ( in volts) across the component.

Must be placed in parallel around the component under test - NOT AROUND THE VARIABLE RESISTOR OR THE BATTERY.

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Five Important Points About the Standard Circuit

Is used for testing components and for getting V-I graphs from them.

The component, the ammeter and the variable resistor are all in series, which means they can be put in any order in the main circuit. The voltmeter, on the other hand, can only be placed in parallel around the component under test.

As you vary the variable resitor it alters the current flowing through the circuit.

This allows you to take several pairs of readings from the ammeter and voltmeter.

You can then plot these values for current and voltage on a V-I graoh and find the resistance.

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Potential Difference-Current Graphs

V-I graphs show how the current varies as you change the potential difference.

DIFFERENT RESISTORS

The current through a resistor (at constant temperature) is directly proportional to P.D. Different resistors have different resistances, therefore there can be different slopes.

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Potential Difference- Current Graphs

FILAMENT LAMP

As the temperature of the filament increases, the resistance increases, hence the curve.

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Potential Difference- Current Graphs

DIODE

Current will only flow through a diode in one direction, as shown. The diode has very high resistance in the opposite direction.

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Resistance Increases with Temperature

When an electrical charge flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot.

This heat energy causes the ions in the conductor to vibrate more. With the ions moving around it is more difficult for the charge-carrying electrons to get through the resistor- the current cannot flow as easily and the resistance increases.

For most resistors there is a limit to the amount of current that can flow. More current means an increase in temperature, which means an increase in temperature, which means an increase in resistance, which means the current decreases again.

This is why the graph for the filament lamp levels off at high currents.

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Resistance, Potential Difference and Current

Potential Difference = Current X Resistance

V= I X R

For the straight line graphs, the resistance of the component is steady and is equal to the inverse of the gradient of the line. In other words, the steeper the graph the lower the resistance.

If the graph curves, it means the resistance is changing. In that case R can be found for any point by taking the pair of values ( V, I ) from the graph and putting them in the formula.

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Resistance, Potential Difference and Current

EXAMPLE : Voltmeter V reads 6V and resistor R is 4 ohms. What is the current through ammeter A?

ANSWER: I = 6 / 4 = 1.5A

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Diode

A diode is a special device made from semiconductor material such as silicon.

It is used to regulate the potential difference in circuits.

It lets current flow freely through it in one direction, but not in the other ( i.e. there's a very high resistance in the reverse direction).

This turns out to be really useful in various electronic circuits.

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Light- Emitting Diodes are Very Useful

A light emitting diode (LED) emits light when a current flows through it in the forward direction.

LED's are being used more and more as lighting as they use a much smaller current than other forms of lighting.

LEDs indicate the presence of current in a circuit. They're often used in appliances (e.g TV's) to show that they are switched on.

They're also used for the numbers on digital clocks, in traffic lights and in remote controls.

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Light Dependent Resistor

An LDR is a resitor that is dependent on the intensity of light.

In bright light, the resistance falls.

In darkness, the resistance is highest.

They have lots of applications including automatic night lights, outdoor lighting and burgular detectors.

Series Circuits

In series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply ( except for voltmeters, which are always connected in parallel, but they do not count as part of the circuit).

If you remove or disconnect one component, the circuit is broken and they all stop.

This is generally not very handy, and in practise very few things are connected in series.

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Potential Difference is Shared

In series circuits the total P.D of the supply is shared between the various components. So the voltages round a series circuit always add up to equal the source voltage:

V=V1+V2+ ...

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Current is the Same Everywhere

In series circuits the same current flows through all parts of the circuit, i.e:

A1=A2

The size of the current is determined by the total P.D. of the cells and the total resistance of the circuit: ie. I=V/R

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Resistance Adds Up

In series circuits the total resistance is just the sum of all the resistances:

R= R1+R2+R3

The bigger the resistance of a component, the bigger its share of the total P.D.

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Cell Voltages Add Up

There is a bigger potential difference when more cells are in series, provided the cells are all connected the same way.

For example when two batteries of voltage 1.5 V are connected in series they supply 3V between them.

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Parallel Circuits

In parallel circuits, each component is seperately connected to the +ve and -ve of the supply.

If you remove or disconnect one of them, it will hardly affect the others at all.

This is how most things must be connected, for example in cars and in household electrics. You have to be able to switch everything on and off seperately.

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P.D is the Same Across All Components

In parallel circuits all components get the full source P.D, so the voltage is the same across all components:

V1=V2=V3

This means that identical bulbs connected in parallel will all be at the same brightness.

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Current is Shared Between Branches

In parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the seperate components.

A=A1+A2+...

In parallel circuit, there are juctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving.

If two identical components are connected in parallel then the same current will flow through each component.

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Voltmeters and Ammeters Are Exceptions to the Rule

Ammeters are always connected in series even in a parallel circuit.

Voltmeters are always connected in parallel with a component even in a series circuit.

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Example of a Series Circuit and Parallel Circuits

Christmas fairy lights are often wired in series.

Everything electrical in a car is connected in parallel.

Parallel connection is essential in a car to give these two features: --> Everything can be turned on and off seperately --> Everything always gets the full voltage from the battery

The only slight effect is that when you tuen lots of things on the lights may go dim because the battery cannot provide full voltage under heavy load. This is normally a very slight effect.

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Mains Electricity

Mains Supply is AC, Battery Supply is DC

The UK mains supply is approximately 230 volts.

It is an AC supply, which means the current is constantly changing direction.

The frequency of the AC mains supply is 50 cycles per second or 50Hz.

By contrast, cells and batteries supply direct current (DC). This just means that the current always keeps flowing in the same direction.

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Mains Electricity

Electricity Supplies Can be Shown on an Oscilloscope Screen

A cathode ray oscilloscope (CRO) is basically a voltmeter.

If you plug an AC supply into an oscilloscope, you get a 'trace' on the screen that shows how the voltage of the supply changes with time. The trace goes up and down in a regular pattern- some of the time it's positive and some of the time it's negative.

If you plug in a DC supply, the trace you get is just a straight line.

The vertical height of the AC trace at any point shows the input voltage at that point. By measuring the height of the trace you can find the potential difference of the AC supply.

For DC it;s a lot simpler - the voltage is just the distance from the straight line trace to the centre line.

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How to Read an Oscilloscope Trace

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Electricity in the Home

Hazards in the Home

Long Cables

Frayed Cables

Cables in contact with something hot or wet

Water near sockets

Shoving things into sockets

Damaged plugs

Too many plugs in one socket

Lighting sockets without bulbs in

Appliances without their covers on

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Most Cables Have Three Separate Wires

Most electriocal appliances are connected to the mains supply by three-core cables. This means that they have three wires inside them, each with a core of copper and a coloured plastic coating.

The brown LIVE WIRE in a mains supply alternates between a HIGH +VE AND -VE VOLTAGE.

The blue NEUTRAL WIRE is always at 0V. Electricity normally flows in and out through the live and neutral wires only.

The green and yellow EARTHWIRE is for protecting the wiring, and for safety- it works together with a fuse to prevent fire and shocks. It is attached to the metal casing of the plug and carries the electricity to earth (and away from you) should something go wrong and the live or neutral wires touch the metal case.

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Three-Pin Plugs and Cables

Wiring

The right coloured wire is connected to each pin, and firmly screwed in.

No bare wires showing inside the plug.

Cable grip tightly fastened over the cable outer layer.

Different appliances need different amounts of electrical energy. Thicker cables have less resistance, so they carry more current.

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Three-Pin Plugs and Cables

Plug Features

The metal parts are made of copper or brass because these are very good conductors.

The case, cable grip and cable insulation are made of rubber or plastic because they're really good insulators, and flexible too.

This all keeps the electricity flowing where it should

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Fuses and Earthing

Earthing and Fuses Prevent Electrical Overloads

The earth wire and fuse (or circuit breaker) are included in electrical appliances for safety and work together like this:

If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, too great a current flows in through the live wire, through the case and out down the earth wire.

This surge in current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating. This cuts off the live supply and breaks the circuit.

This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents the risk of fire caused by the heating effect of a large current.

As well as people, fuses and earthng are there to protect the circuits and wiring in your appliances from getting hot if there is a current surge.

Fuses should be rated as near as possible but just higher than the normal operating current.

The larger the current, the thicker the cable you need to carry it. That's why the fuse rating needed for cables usually increases with cable thickness.

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Insulating Materials Make Appliances Double Insula

All appliances with metal cases are usually 'earthed' to reduce the danger of electric shock.

Earthing just means the case must be attached to an earth wire. An earthed conductor can never become live.

If the appliance has a plastic coating and no metal parts showing then it's said to be double insulated.

Anything with double insulation like that doesnt need an earth wire- just a live and neutral.

Cables that only carry the live and neutral wires are known as two-core cables.

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Circuit Breakers

Circuit breakers are an electrical safety device used in some circuits. Like fuses, they protect the circuit from damage if too much current flows.

When circuit breakers detect a surge in current in a circuit. they break a circuit by opening a switch.

A circuit breaker (and the circuit theyre in) can easily be reset by flicking a switch on the device. This makes them more convenient than fuses- which have to be replaced once they've melted.

They are, however, a lot more expensive to buy than fuses.

One type of circuit breaker used instead of a fuse and an earth wire is a Residential Current Circuit Breakers (RCCB's)

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RCCB's

Normally exactly the same current flows through the live and neutral wires. If somebody touches the live wire, a small but deadly current will flow through them to the earth. This means the neutral wire carries less current than the live wire. The RCCB detects this difference in current and quickly cuts off the power by opening a switch.

They also operate much faster than fuses- they break the circuit as soon as there is a current surge- no time is wasted waiting for the current to melt the fuse. This makes them safer.

RCCB's even work for small current changes that might not be large enough to melt a fuse. Since even small current changes could be fatal, this means RCCB's are more effective at protecting against electrocution.

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Energy and Power in Circuits

Energy is Transferred from Cells and Other Sources

Anything which supplies electricity is also supplying energy.

So cells, batteries, generators, etc. all transfer energy to components in the circuit:

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Resistors Produce Heat

All resistors produce heat when a current flows through them

Whenever a current flows through anything with electrical resistance ( which is pretty much everything) then electrical energy is converted into heat energy.

The more current that flows, the more heat is produced.

A bigger voltage mean more heating because it pushes more current through.

Filament bulbs work by passing a current through a very thin wire, heating it up so much that it glows. Rather obviously, they waste a lot of energy as heat.

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Efficiency

When you buy electrical appliances you can choose to buy ones that are more energy efficient.

These appliances transfer more of their total electrical energy output to useful energy.

For example, less energy is wasted as heat in power-saving lamps such as compact fluorescent lamps (CFL's) and light emitting diodes than in ordinary filament bulbs.

Unfortunately, they do cost more to buy, but over time the money you save on your electricity bills pays you back for the initial investment.

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Power Ratings of Appliances

The total energy transferred by an appliance depends on how long the appliance is on and its power rating. The power of an appliance is the energy that it uses per second.

Energy Transferred= Power rating x time

For example: if a 2.5kW kettle is on for 5 minutes, the energy transferred by the kettle in this time is 300x2500=750000J=750kJ

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Electrical Power and Fuse Ratings

The formula for electrical power is: POWER=CURRENTxPOTENTIAL DIFFERENCE

Most electrical goods show their power rating and voltage rating. To work out th size of the fuse needed, you need to work out the current that the item will normally use.

EXAMPLE: A hair dryer is rated at 230 v, 1kW. Find the fuse needed.

ANSWER: I=P/V = 1000/230 = 4.3 A. Normally the fuse should be rated just a little higher than the normal current, so a 5 amp fuse is ideal for this one.